Sheesh,
Actually, fat can potentiate insulin. That is, while fat in and of itself has no impact on insulin levels, in the presence of already elevated insulin, fatty acids can synergistically enhance the insulin response. This phenomenon may be related to gastric inhibitory peptide.
That being said, the adverse effects of caffeine on carbohydrate metabolism are not known for certain, but it's possible that they are mediated by the catecholamines. Since the latter directly affects carbohydrate metabolism and also acts antagonistically to insulin, it's a prime candidate.